It has been known that an activity of oxygen within molten glass in a glass manufacturing process largely affects color, ultraviolet absorptivity, and the like of the produced glass for example. Although an apparatus which allows the activity of oxygen within the molten glass to be continuously measured and its result to be reflected in the manufacturing process has been expected, it is the present state unfortunately that no such apparatus which meets with such expectation has been provided.
There has been known an apparatus for measuring an activity of oxygen within molten metal. Such apparatus comprises a tubular shell which is made of solid electrolyte such as zirconia, whose one end is closed and which is to be soaked into a molten substance to be measured, a reference pole which is provided at an inner face of the shell and which is made of a reference substance which gives a predetermined activity of oxygen, an inner electrode inserted to the reference pole, an outer electrode made of a metal rod to be soaked into the molten substance to be measured and a sheath thermocouple which is to be soaked into the molten substance to be measured to measure temperature of the molten substance.
In measuring the activity of oxygen within the molten metal by the known apparatus, the tubular shell, the outer electrode, and the sheath thermocouple are soaked into the molten metal, respectively. An oxygen concentration cell which generates electromotive force E(V) based on a difference of partial pressure of oxygen existing between the molten substance and the reference pole is formed between the outer and inner faces of the shell made of the solid electrolyte. This electromotive force E(V) is measured by electromotive force measuring means connected to the outer and inner electrodes. The temperature T(K) of the molten substance is measured by the sheath thermocouple.
Values of the electromotive force E(V) and the temperature T(K) thus measured are converted by the following conversion expression 1 to find an amount of dissolved oxygen (activity of oxygen) within the molten metal: Expression 1! EQU a.sub.0 =exp-.DELTA.G.degree./RT!.multidot.(P.theta..sup.1/4 +Pref.sup.1/4)exp-EF/RT!-P.theta..sup.1/4!.sup.2
where, "a.sub.0 " is an activity of oxygen within molten metal; "E" is electromotive force (V); "T" is absolute temperature (K); "R" is gas constant; "F" is Faraday constant; "P.theta." is partial pressure of oxygen at which ion conduction becomes equal to electron conduction; "Pref" is equilibrium partial pressure of oxygen of a reference substance; and ".DELTA.G.degree." is change in free energy accompanying to dissolution into molten metal.
Although it is possible to meet with the expectation described above by applying the above-mentioned apparatus for measuring an activity of oxygen within molten metal to the measurement of an activity of oxygen within molten glass, there are still many problems to be solved.
That is, it is necessary to consider a problem peculiar to a molten substance to be measured in developing the applied technology described above. For instance, there is a problem that when the shell is soaked into the molten glass, stabilized zirconia reacts with and eroded by the molten glass, making it difficult to measure continuously for a long period of time. In this regard, although it is preferable to use stabilized zirconia of, a ZrO.sub.2 -MgO system, a ZrO.sub.2 -CaO system, a ZrO.sub.2 -Y.sub.2 O.sub.3 system, and the like as a zirconia solid electrolyte composing the shell, it has a limit in usable time in terms of corrosion resistance and heat and shock resistance and cannot be used continuously as desired (First Problem).
Further, while the outer electrode has been provided separately from the solid electrolyte as seen in the apparatus for measuring an activity of oxygen within molten metal, there has been a problem when the apparatus thus constructed is used in molten glass that the molten glass between the outer electrode and the solid electrolyte acts as an electrolyte, so that electromotive force varies depending on the size of the outer electrode, a distance between the outer electrode and the solid electrolyte and a positional relationship of the outer electrode and the solid electrolyte with respect to the fluidity of the molten glass. Therefore, the reliability of the measuring accuracy have not been satisfactory (Second Problem).
Further, there has been known a technology of filling mixed powder of Mo-MoO2, of Cr-Cr.sub.2 O.sub.3, or of Ni-NiO within the end of the shell made of the solid electrolyte in order to form a reference pole in the prior art apparatus for measuring an activity of oxygen within molten metal, such mixed powder is not suitable for the continuous measurement because it tends to be sintered and shrink in high temperature in using the apparatus, causing a gap between the solid electrolyte, and is not stable in the long run (Third Problem).
Further, although an arrangement in which the oxygen activity measuring apparatus is provided also with a temperature measuring apparatus has been adopted in the past because it is also necessary to measure temperature of the molten substance in measuring the activity of oxygen utilizing the solid electrolyte, there has been a problem because of the structure in which they are both provided that not only the apparatus cannot be compact as a whole, but also a pair of thermocouple element wires for the temperature measuring means and a pair of lead wires connected with the outer and inner electrodes for the electromotive force measuring means need to be wired, thus complicating the wiring and hampering the reduction of the cost of the whole apparatus (Fourth Problem).